Improve performance of binary COPY FROM through better buffering.

 Improve performance of binary COPY FROM through better buffering.
 
 At least on Linux and macOS, fread() turns out to have far higher
 per-call overhead than one could wish.  Reading 64KB of data at a time
 and then parceling it out with our own memcpy logic makes binary COPY
 from a file significantly faster --- around 30% in simple testing for
 cases with narrow text columns (on Linux ... even more on macOS).
 
 In binary COPY from frontend, there's no per-call fread(), and this
 patch introduces an extra layer of memcpy'ing, but it still manages
 to eke out a small win.  Apparently, the control-logic overhead in
 CopyGetData() is enough to be worth avoiding for small fetches.
 
 Bharath Rupireddy and Amit Langote, reviewed by Vignesh C,
 cosmetic tweaks by me
 
 Discussion: https://postgr.es/m/CALj2ACU5Bz06HWLwqSzNMN=Gupoj6Rcn_QVC+k070V4em9wu=A@mail.gmail.com